Peter Mandelson is a unique figure in British politics. It is not only his extraordinary capacity to return from apparent political bankruptcy to the very top of a party in parts of which he is still cordially loathed. His admirers across the political spectrum say that his is probably the most subtle and creative political brain of his generation. But for a significant section in the party, he is the man who accommodated Thatcherism; who lives his own mantra of being "intensely relaxed" about the very rich with whom he holidays (again, this summer); and who is intimately associated with a pro-business agenda.

In his 25 years as a major player, Lord Mandelson's critics would claim, he has never displayed a sense of a cause beyond the Labour brand – and himself. By reinventing the brand, and divorcing it from its cause, he has changed the shape of contemporary politics. It is now a politics that is overtly about power rather than people: a politics that is arguably more like 18th-century factionalism than the ideological politics of the 20th century. It is a technocratic politics. In our interview today, there is a telling moment where he contemptuously ignores a fellow passenger against whose telephone manners he has, with barely a lifted eyebrow, united most of the carriage. A human response, perhaps – but hardly humane.

Turning the Labour party round from its impassioned but suicidal "purification" debates of the late 1970s – rescuing it from what he understood to be historic decline – required a repositioning that was not going to be volunteered by some existing Labour activists. Along with Neil Kinnock, Mandelson was the key player in the long campaign to make Labour palatable to the post-Thatcher middle classes. It is possible – with Mandelson, it sometimes seems anything is possible – that we are now about to meet a man of ideology. There have been recent nods to tunes from the old Labour hymnal in his attack on universities' admissions policies and his talk of industrial activism. Admirers would say that this is the real Mandelson. His talent is an ability to understand complex, long-term problems (Northern Ireland and British industry, as well as Labour's decline) and to put together alliances that – inch by inch, clause by sub-clause – allow him to reach his desired destination. The interesting question is quite where he thinks he is going himself.

His emergence at the side of a beleaguered prime minister has given him unprecedented power. To his detractors, this is a new move to keep the party in the business of government, regardless of cost. To his admirers, it is proof that, beyond everything, he is a party man.